High-speed radiation detection devices are of interest in a variety of fields such as, e.g., optical communications, image sensing, and measurement and instrumentation; devices may be sensitive to particle radiation or, as in the following, to electromagnetic radiation. Of particular interest are wavelength-selective devices which are capable of following a rapidly changing signal such as, e.g., a train of pulses in which pulse duration is on the order of a few tens of picoseconds.
In the case of infrared radiation, high-speed detector devices are particularly useful, e.g., for satellite communications, terrain mapping, and infrared viewing. Devices in current use in these fields of application are largely based on the material mercury cadmium telluride, cooled with liquid nitrogen. However, this material is not easy to fabricate, and the material may be unsatisfactory with respect to long-term stability. While doped silicon also represents a possible infrared-radiation-sensitive medium, cooling to still lower temperatures is required. Also, the response of silicon-based devices is considered to be unsatisfactory for high-speed applications.
Experimental devices have been proposed as predicted on optoelectronic effects in compound-semiconductor materials, Group II-VI and Group III-V materials being considered as particularly suitable in this respect. For example, the paper by D. D. Coon et al., "New Mode of IR Detection Using Quantum Wells", Applied Physics Letters, Vol. 45 (1984), pp. 649-651 discloses infrared radiation detection based on charge-depletion in localized impurity levels in semiconductors to which an electric field is applied. Charge-depletion takes the form of photoemission from a single Al.sub.x Ga.sub.1-x As/GaAs/Al.sub.y Ga.sub.1-y As asymmetric quantum well.
Use of a plurality of GaAs/GaAlAs quantum wells is disclosed in the paper by J. S. Smith et al., "A New Infrared Detector Using Electron Emission from Multiple Quantum Wells", Journal of Vacuum Science and Technology, Vol. B1 (1983), pp. 376-378. Here, electrons are ejected from quantum wells upon excitation by free-carrier absorption, giving rise to a flow of electric current.
A superlattice consisting of a plurality of GaAs/Ga.sub.x Al.sub.1-x As quantum wells is disclosed also in the paper by L. Esaki et al., New Photoconductor", IBM Technical Disclosure Bulletin, Vol. 20 (1977), pp. 2456-2457. In the disclosed structure, electrons in the lowest sub-band of quantum wells are essentially immobile, while electrons in a second subband have significant mobility.